1. Field of the Invention
The present invention relates to an electrolyte solution and lithium-ion batteries having the same, and more particularly, to an electrolyte solution having an ionic liquid and a lithium-ion battery having the same.
2. Description of Related Art
Due to the application in green cars and energy storage systems, the market share of lithium-ion batteries increases to 20% from 2.5%. It has been predicted by Zpryme Research & Consulting, LLC that the global smart grid market would be 171.4 billion dollars. Pike Research has predicted that in 2015, there would be 32,000 buses using green energy in the world, and thus there would be huge demand for lithium-ion batteries. Therefore, it is important to design novel lithium-ion batteries with thermal resistance.
The safety of batteries is critical in the development of electric vehicles. Currently, the most widely used organic electrolyte solution is flammable. In the operation of the batteries, there may be abnormal voltage or lost control of charge/discharge, thus oxygen is released from the anode material, and highly reactive ions produced from decomposition of the electrolyte solution. Once the temperature is increased, the batteries may explore. The organic electrolyte solution having electrolyte and complicated components, and may be decomposed at 60 to 100° C. Therefore, it is an important issue to develop an electrolyte solution with thermal resistance and high voltage tolerance to improve safety of lithium-ion batteries, in which an ionic liquid having specific property may be used. Currently, it is limited that an ionic liquid is used in an electrolyte solution. It has been found that an ionic liquid has the property such as extreme low vapor pressure, low melting point, high polarity, non-flammability, strong acid resistance at room temperature. An ionic liquid is used in an electrolyte solution of lithium-ion batteries to improve thermal stability of the batteries in the prior art. For example, WO2007/104144 A1 discloses an anion structure:
which may form an ionic liquid with the following cations:

In the conventional lithium-ion batteries, the ionic liquid added into the electrolyte solution is more than 50 wt %, so as to improve thermal resistance. However, the ionic liquid has higher viscosity than the common electrolyte solution, such that the ionic liquid increases the number of conductive ions but retards the movement of the ions. In the prior art, the battery capacity significantly decreases at a high discharge rate, and thus the solvent in the common electrolyte solution cannot be substituted by the ionic liquid. Hence, it is to be developed in the present invention that an ionic liquid is added in a small amount into the common electrolyte solution for significantly improving thermal stability of the electrolyte solution.
It has been disclosed in the prior art that the amount of an ionic liquid added in an electrolyte solution is less than 50 wt %. For example, Kühnel et al disclose that 1-butyl-1-methylpyrrolidium bis(trifluoromethylsulfonyl)imide (PYR14TFSI) and poly(carbonate) are mixed at different ratios, and the mixture is analyzed with LiTFSI in a thermogravimetric analysis. (Electrochimica Acta, 56, p 4092-4099, 2011). The result shows that when the amount of the ionic liquid is 20 wt %, the electrolyte solution has significant thermal reaction even at 50° C. In addition, the conductivity of the electrolyte solution having 2 wt % of PYR14TFSI increases about 0.25 mS/cm. Further, Zaghib et al disclose that the conductivity of an electrolyte solution is increased by the addition of an ionic liquid, wherein (Journal of Power Sources, 195, 845-852, 2010) 2 wt % of [EMI]TFSI[EMI]TFSI is added into EC-DEC-VC electrolyte solution having 1M of LiPF6, and then the conductivity of the electrolyte solution only increases about 0.1 mS/cm. In the thermogravimetric analysis, even 30 wt % of [EMI]TFSI[EMI]TFSI is added, the thermal reaction occurs at about 50° C., just like the electrolyte solution without the ionic liquid.
In addition, the current ionic liquid mainly has anionic groups with TFSI, which may be decomposed at high voltage and applicable at the electrochemical window less than 3.9V, so as to limit the applications of lithium-ion batteries in electric vehicles and electric tools. Most developments are provided to improve and increase electron withdrawing groups such as F, Cl or CN in cationic groups.
Accordingly, in order to overcome the above-mentioned drawbacks in the conventional lithium-ion battery and the conventional electrolyte solution, there is a need to develop an ionic liquid having high conductivity and high thermal stability and to develop an electrolyte solution with advantages of an organic electrolyte solution and an ionic liquid.